Due in part to global warming and climate change there is a growing interest in the use of carbon dioxide from such sources as carbon capture and storage (CCS), carbon dioxide capture from flue gases or carbon dioxide waste from industrial processes such as brewing, in the manufacture of hydrocarbons. These sources of carbon dioxide have been considered for use in combination with hydrogen obtained from water electrolysis using renewable sources of energy although in principle the hydrogen could be sourced from waste streams from conventional petrochemical processes or other sources.
One well know process for the manufacture of hydrocarbons is the Fischer-Tropsch, which converts carbon monoxide and hydrogen to hydrocarbons typically over a cobalt or iron catalyst. The usual source of carbon monoxide and hydrogen is synthesis gas or syngas.
Generally, the Fischer-Tropsch process is operated in the temperature range of 150-300° C. (302-572° F.). Higher temperatures lead to faster reactions and higher conversion rates but also tend to increase methane production. As a result, the temperature is usually maintained at the low to middle part of the range. Increasing the pressure leads to higher conversion rates and also favors formation of long-chained alkanes both of which are desirable. Typical pressures range from one to several tens of atmospheres.
A variety of synthesis gas compositions can be used. For cobalt-based catalysts the optimal H2:CO ratio is around 1.8-2.1. Iron-based catalysts promote the water-gas-shift reaction and thus can tolerate significantly lower ratios. This reactivity can be important for synthesis gas derived from coal or biomass, which tend to have relatively low H2:CO ratios (<1).
The conversion of carbon dioxide to hydrocarbons via hydrogenation has been know for a number of years as described for example in U.S. Pat. No. 2,692,274. In recent years there has been increasing interest given to using carbon dioxide in combination with hydrogen as a feed mixture for a Fischer-Tropsch type process with a reverse water gas shift reaction, however these emerging processes for the utilisation of carbon dioxide/hydrogen feeds are not, as yet, optimized processes and have the problems and challenges that are typically associated with such catalytic processes.
In US2005232833A1 there is described a process for producing synthetic hydrocarbons that reacts carbon dioxide, obtained from seawater of air, and hydrogen obtained from water, with a catalyst in a chemical process such as reverse water gas shift combined with Fischer Tropsch synthesis. The hydrogen is produced by nuclear reactor electricity, nuclear waste heat conversion, ocean thermal energy conversion, or any other source that is fossil fuel-free, such as wind or wave energy. The process can be either land based or sea based.
In US2008051478A1 there is described a method and apparatus of introducing hydrogen and a feed gas containing at least 50 vol % carbon dioxide into a reactor containing a Fischer-Tropsch catalyst; and heating the hydrogen and carbon dioxide to a temperature of at least about 190° C. to produce hydrocarbons in the reactor.
In WO2010002469A1 there is described a system for converting carbon dioxide into a fuel to be reburned in an industrial process. The preferred feed stocks are taken from large volume carbon dioxide producers, and municipal waste. The reaction and processes reclaim lost energy in municipal waste, and industrial exhaust gas. The system is provided with a plasma melter having a feedstock input for receiving a feed fuel, and a syngas output for producing a syngas having an H2 component. Additionally, a Sabatier reactor is provided having a hydrogen input for receiving at least a portion of the H2 component produced by the plasma melter, and a methane output for producing CH4.
In US2010111783 there is described a process and system for producing hydrocarbon compounds or fuels that recycle products of hydrocarbon compound combustion-carbon dioxide or carbon monoxide, or both, and water. The energy for recycling is electricity derived from preferably not fossil based fuels, like from nuclear fuels or from renewable energy. The process comprises electrolysing water, and then using hydrogen to reduce externally supplied carbon dioxide to carbon monoxide, then using so produced carbon monoxide together with any externally supplied carbon monoxide and hydrogen in Fischer-Tropsch reactors, with upstream upgrading to desired specification fuels-for example, gasoline, jet fuel, kerosene, diesel fuel, and others. Energy released in some of these processes is used by other processes. Using adiabatic temperature changes and isothermal pressure changes for gas processing and separation, large amounts of required energy are internally recycled using electric and heat distribution lines. Phase conversion of working fluid is used in heat distribution lines for increased energy efficiency. The resulting use of electric energy is less than 1.4 times the amount of the high heating value of combustion of so produced hydrocarbon compounds when carbon dioxide is converted to carbon monoxide in the invention, and less than 0.84 when carbon monoxide is the source.
In GB2461723A there is described a process where carbon dioxide gas exhausted from power stations is collected by means of absorption into an absorptive fluid. The carbon dioxide is used as the carbon component for hydrocarbon or alcohol fuel. In another section, water is separated into its constituent elements, namely hydrogen and oxygen, by electrolysis. The hydrogen is combined with the carbon dioxide in an exothermic reaction to produce methanol. Methanol is a preferred automobile fuel as the pure substance or as a mixture with conventional motor fuel. If it is desired, the methanol may be converted to ethanol or conventional motor fuel. The process represents an energy conversion technology, since the carbon dioxide has no reductive (calorific) heat value, and is essentially inert.
In WO2008115933A there is described a Renewable Fischer Tropsch Synthesis (RFTS) process, which produces hydrocarbons and alcohol fuels from wind energy, waste CO2 and water. The process includes (A) electrolyzing water to generate hydrogen and oxygen, (B) generating syngas in a reverse water gas shift (RWGS) reactor, (C) driving the RWGS reaction to the right by condensing water from the RWGS products and separating CO using a CuAlCl4-aromatic complexing method, (D) using a compressor with variable stator nozzles, (E) carrying out the FTS reactions in a high-temperature multi-tubular reactor, (F) separating the FTS products using high-pressure fractional condensation, (G) separating CO2 from product streams for recycling through the RWGS reactor, and (H) using control methods to maintain temperatures of the reactors, electrolyzer, and condensers at optima that are functions of the flow rate. The RFTS process may also include heat engines, a refrigeration cycle utilizing compressed oxygen, and a dual-source organic Rankine cycle.
A major challenge in many catalytic processes is indentifying operating conditions, which ensure optimum utilization of the catalysts and/or process conditions. Catalysts have a useful life and must eventually be replaced or reconditioned in order to keep the process operating at the optimum conditions. The initial conditioning of the catalyst, the start-up conditions and ongoing operating conditions all have an impact on overall catalyst performance. Through any given cycle the catalyst activity will diminish and this is often compensated by changing the process to conditions that are even harsher on the catalysts resulting in accelerated catalyst deactivation. There is a typical tradeoff between the costs of catalyst replacement/reconditioning compared to the increased running costs to maintain activity.
This is a particular problem for catalysts used in processes for the conversion of carbon dioxide/hydrogen where harsh conditions may be required and catalyst life is shortened as a consequence.